Internal hydrogen effects on tensile properties of iron- and nickel-base superalloys
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I.
INTRODUCTION
HYDROGENembrittlement of
austenitic stainless steels and superalloys has been studied since the early 1960's, but much of the work on superalloys has emphasized testing in H-containing environments. It was therefore decided to initiate a study of the effects of internal H on two Ni-base alloys (INCONEL* 718 and INCONEL 625) *INCONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.
and one Fe-base alloy (A286). INCONEL 718 (IN718) is a precipitation-hardened alloy with good ductility and excellent strength from - 2 5 0 ~ to 650 ~ It is produced as a wrought, weldable alloy and as casting s. Gamma prime (3") and gamma double prime (~/") precipitates containing a considerable amount of columbium, molybdenum, titanium, and aluminum give this alloy its strength. The slow diffusion rate of the elements causes a sluggish precipitationhardening reaction which permits annealing and welding without hardening during heating and cooling. The alloy is widely used in gas turbine engines for temperatures up to 700 ~ but it has also found application in liquid H rocket technology. Exposure of the material to H2 gas at high temperature can result in H absorption, leading to the possibility of room-temperature embrittlement due to the dissolved H. At room temperature, IN718 is susceptible to delayed failure and reduced tensile properties in gaseous H 2 at pressures only in excess of 7 MPa. u-9] After nonuniform, high-temperature charging, there was no consistent correlation between the degree of embrittlement and the strength level for IN718 (various heat treatments), t9] The following study will look more closely at the behavior of this alloy when it is uniformly charged with H. Superalloy A286 is a 3/' precipitation-hardened iron-base alloy used primarily for jet engine parts requiring high P.D. HICKS, Research Assistant, and C.J. ALTSTETIER, Professor of Physical Metallurgy, are with the Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted May 26, 1989. METALLURGICAL TRANSACTIONS A
strength up to 700 ~ and oxidation resistance up to 815 ~ Only minor hydrogen embrittlement of A286 occurs at temperatures above ambient, and aging of A286 increases its sensitivity to H. tu Some investigators have found that the tensile fracture and sustained load toughness test results for A286 are not affected by external H2 pressures as high as 52 MPa, t2l but after very severe thermal charging, slow strain rate tests at 0 ~ have shown a loss in reduction in area of 46 pct.[9] INCONEL 625 is a solid solution-strengthened Ni-base alloy combining strength, weldability, and resistance to a wide range of corrosive environments. It has thus found application in seawater, aerospace, and nuclear environments. Tensile properties of plate tested in 35 MPa H2 gas were negligibly degraded, at temperatures above 225 ~ i . e . , less than 10 pct reduction when compared to similar tests in a helium gas environment, t~~ H a r r i s Ul] showed that a
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